The separation of low-boiling gas mixtures requires refrigeration to cool and condense feed streams which are separated into desired product fractions. Refrigeration typically is provided to a warm feed stream by indirect heat exchange with cold process streams from other locations within a process plant, and also can be provided by closed-loop or recirculating refrigerant systems. Refrigeration also can be generated by work expansion of the feed stream itself, portions of the feed stream, or other process streams.
Refrigeration is provided by these methods for the separation of feed gas mixtures contain light gases such as hydrogen, nitrogen, carbon monoxide, and carbon dioxide as well as light hydrocarbons containing one or more carbon atoms. Typical applications include nitrogen rejection from nitrogen-light hydrocarbon mixtures and ethylene or propylene recovery from cracked gases produced by hydrocarbon pyrolysis.
The cooling and condensing of a feed gas in these applications typically involves at least one multiple-stream conventional heat exchanger or dephlegmator in which refrigeration is provided by three or more cold process streams and possibly by dedicated refrigerant streams. This requires a complex heat exchanger with significant piping and manifolding for the introduction and withdrawal of the numerous warmed and cooled steams. In many cases, the flow rate of the feed gas stream exceeds the capacity of a single heat exchanger, and multiple parallel heat exchangers must be used. This further complicates the required piping and manifolding.
In process situations which require such a plurality of large, multiple-stream heat exchangers, it is desirable to simplify the required piping and manifolding in order to reduce capital cost and operating complexity. The invention described below and defined in the claims which follow addresses this need with a simplified method and system for cooling process stream with multiple refrigerant streams.
The invention is a method for cooling a process stream which comprises obtaining a plurality of open-loop refrigerant streams, selecting one of the open-loop refrigerant streams as a primary refrigerant stream, and cooling the primary refrigerant stream by indirect heat exchange with other of the open-loop refrigerant streams in a refrigerant heat exchanger to yield a cooled primary refrigerant stream. The process stream is cooled by indirect heat exchange with a process refrigerant stream in a process heat exchanger to yield one or more cooled process streams and a warmed process refrigerant stream, wherein the process refrigerant stream comprises at least a portion of the cooled primary refrigerant stream.
The method can further comprise operating one or more additional process heat exchangers in parallel with the process heat exchanger wherein each of the additional process heat exchangers cools an additional process stream by indirect heat exchange with a portion of the process refrigerant stream. The process stream and the additional process streams typically are gas mixtures which have identical compositions.
The process heat exchanger can be a dephlegmator heat exchanger which cools, partially condenses, and rectifies a mixed feed gas stream to produce a condensate stream and a light overhead gas stream. Alternatively, the process heat exchanger can be a conventional heat exchanger.
The warmed process refrigerant stream optionally can be work expanded to yield an expanded cooled refrigerant stream which provides one of the other open-loop refrigerant stream. The expanded cooled refrigerant stream optionally can be warmed to provide refrigeration in the refrigerant heat exchanger, a warmed expanded refrigerant stream can be with drawn therefrom, and the warmed expanded refrigerant stream can be further work expanded to yield a further expanded cooled refrigerant stream which provides another of the other open-loop refrigerant streams.
A portion of the light overhead gas stream can be combined with the cooled primary refrigerant to provide the process refrigerant stream. The warmed process refrigerant stream an be work expanded to yield an expanded cooled refrigerant stream which can provide one of the other open-loop refrigerant streams.
The expanded cooled refrigerant stream can be warmed to provide refrigeration in the refrigerant heat exchanger, a warmed expanded refrigerant stream can be withdrawn therefrom, and the warmed expanded refrigerant stream can be further work expanded to yield a further expanded cooled refrigerant stream which provides another of the other open-loop refrigerant streams.
The cooled primary refrigerant stream can be a two-phase vapor-liquid primary refrigerant stream. In this embodiment, the method further comprises
(1) separating the two-phase vapor-liquid primary refrigerant stream into a vapor refrigerant stream and a liquid refrigerant stream;
(2) combining the vapor refrigerant stream with a portion of the light overhead gas stream to yield a combined vapor refrigerant;
(3) warming the combined vapor refrigerant in the refrigerant heat exchanger to provide refrigeration therein and yield a warmed combined vapor refrigerant;
(4) work expanding the warmed combined vapor refrigerant to provide a cooled combined refrigerant; and
(5) combining the cooled combined refrigerant with the liquid refrigerant stream of (1) to provide the process refrigerant stream.
The method can further comprise withdrawing the warmed process refrigerant stream from the process heat exchanger and work expanding the warmed process refrigerant to provide one of the other open-loop refrigerant streams.
The process stream cooled by the method of the present invention can comprise hydrogen, methane, ethylene, and ethane. Alternatively, the process stream can comprise nitrogen, ethane, and ethane. In another alternative, the process stream can comprise hydrogen carbon monoxide, and methane.
The light overhead gas stream can comprise hydrogen and methane. In this embodiment, the method can further comprise separating a portion of the light overhead gas stream into a hydrogen-enriched product stream and a methane-enriched fuel stream, and utilizing the hydrogen-enriched product stream and the methane-enriched fuel stream to provide two of the other open-loop refrigerant streams.
If desired, two of the plurality of open-loop refrigerant streams can be combined to provide the primary refrigerant stream which is cooled by indirect heat exchange with the other open-loop refrigerant streams.
Additional refrigeration, if required, can be provided to either or both of the refrigerant heat exchanger and the process heat exchanger by warming a refrigerant provided by a closed-loop refrigeration system.
The invention also includes a heat exchanger system for cooling a process stream which comprises:
(a) refrigerant heat exchange means for providing indirect heat exchange among a plurality of open-loop refrigerant streams whereby one of the open-loop refrigerant streams is cooled by indirect heat exchange with other of the open-loop refrigerant streams to yield a cooled primary refrigerant stream and warmed refrigerant streams;
(b) process heat exchange means for providing indirect heat exchange between at the process stream and a process refrigerant stream to yield a cooled process stream and an additional warmed refrigerant stream, wherein the process refrigerant stream comprises at least a portion of the cooled primary refrigerant stream.
(c)piping means for introducing the open-loop refrigerant streams into the refrigerant heat exchange means and for withdrawing the cooled primary refrigerant stream and warmed refrigerant streams from the refrigerant heat exchange means; and
(d) piping means for introducing the process refrigerant stream into the process heat exchange means and for withdrawing the cooled process stream and the warmed process refrigerant stream from the process heat exchange means.